1. Field of the Invention
The present invention generally relates to an image forming apparatus having a recording head for ejecting recording liquid droplets.
2. Description of the Related Art
As for known image forming apparatuses, there are printers, facsimile machines, copiers, plotters, and multi-function machines. One example of such an image forming apparatus is a liquid jet recording apparatus (e.g., inkjet recording apparatus). The liquid jet recording apparatus performs an image forming (also referred to as “recording”, “printing” and the like) operation by using a recording head that ejects droplets of ink to a sheet. The liquid jet recording apparatus includes, for example, a serial type image forming apparatus that forms images by ejecting liquid droplets while moving the recording head in a main scanning direction or a line type image forming apparatus that forms images by ejecting liquid droplets without moving the recording head.
It is to be noted that the term “image forming apparatus” includes apparatuses that form images by impacting ink to materials such as paper, string, fiber, fabric, leather, metal, plastic, glass, wood, and ceramic. Furthermore, the term “image formation” not only includes forming images that have a meaning (e.g., letters, shapes) on a medium but also includes forming images having no particular meaning (e.g., patterns. i.e., liquid droplets are just impacted to a medium). Furthermore, the term “ink” not only includes so-called ink, but also includes any liquid which can be used to form images (e.g., recording liquid, fixing liquid, etc.). Furthermore, the term “sheet” not only includes paper, but also includes any materials onto which ink droplets can adhere (e.g., sheet for overhead projector, fabric, so-called recording medium or recording sheet and the like).
In such image forming apparatuses, a sheet detection sensor is provided so as to detect a leading edge of a sheet or a position of a sheet with respect to a direction along the width of the sheet. A blank space (nonprintable area) in the trailing edge of the sheet is determined based on the sheet size and the position of the leading edge of the sheet detected by the sheet detection sensor. In other words, an area for image forming is determined.
In apparatuses using a belt as a sheet transportation unit, it is difficult to use a mechanical switch or a transmissive photo sensor in order to detect a leading edge of a sheet because the sheet is placed and transported on the belt. As a technique concerning this, Japanese Patent Laid-Open Publication No. 2006-082231 (Patent Document 1) and 2005-007799 (Patent Document 2) disclose that a reflective photo sensor is used so as to detect a sheet. In apparatuses of an electrophotographic system, Japanese Patent Laid-Open Publication No. 2002-362775 (Patent Document 3) is related.
However, in some kinds of sheets, a reflected light of the sheet for incident light from a reflective photo sensor is not so different from a reflected light of the surface of the belt (e.g., sheet for overhead projector, thin paper, colored paper). In such sheets, it is not able to detect a leading edge of the sheet correctly by a reflective photo sensor.
Therefore, if such a reflective photo sensor mentioned in Patent Document 1 or 2 is used, an image forming area of the sheet (i.e., blank space in a leading edge or a trailing edge) is not determined correctly based on a leading edge position detected by the reflective photo sensor. Because of this, liquid droplets are ejected on the belt and it causes a problem that a surface of a following sheet is stained by the liquid deposited on the belt.